The crankshaft sensors are used in a motor vehicle in order to determine the position of the crankshaft and the speed of rotation and the direction of rotation of the engine. Used in combination with camshaft sensors, they determine the position of the different cylinders in the combustion cycle of the engine (i.e. determine for each cylinder whether said cylinder is in an intake phase, compression phase, explosion phase, or exhaust phase) and make it possible to manage the operation of the engine to the best possible extent by optimal regulation of the spark timing or the moment of fuel injection.
These crankshaft sensors comprise a magnetic field generator (for example: a permanent magnet), a magnetic field detection means (Hall effect cell, magnetoresistive (MR) cell, giant magnetoresistive (GMR) cell, etc., for example) and an electronic circuit for processing the signal received by the magnetic field detection means. These sensors, referred to as active sensors, deliver a digital signal to a central computer for processing.
The magnetic field generator may also be a target, made of a magnetic material, having alternating south and north poles. In this case the sensor may or may not include a permanent magnet depending on the detection means used. Consequently, the south and north poles will equate to the teeth and troughs of a mechanical target.
As illustrated in FIG. 1, a crankshaft sensor 10 is associated with a target 14 secured to a crankshaft 16. This target 14 is in the form of a disc 15 of which the periphery is toothed. A space (trough) C1, C2, C3 is located between each tooth T1, T2, T3, said teeth being substantially identical. The target is distinguished by the presence of a trough Ce of greater length, referred to more commonly as a “missing tooth” positioned precisely at a certain angle with respect to the angular position of the engine. In accordance with FIG. 1, a crankshaft sensor 10 comprises, as is known, a ferromagnetic element 11 and a magnetic field detection means 12 (for example a Hall effect sensor). This sensor 10 delivers a digital signal to one of the processing means 13.
The operation of such a sensor assembly 10 and of the associated target 14 is described hereinafter.
When the target 14 is driven in rotation (arrow F FIG. 1) by the crankshaft 16, the sensor 10 perceives a series of variations of the magnetic field representative of the tooth or teeth T1, T2, T3 passing in front of said sensor and also representative of the spacing thereof C1, C2, C3, Ce. This signal thus obtained is shown in FIG. 2.
FIG. 2 shows, in accordance with the prior art, the signal B of the magnetic field delivered by the sensor 10 on the basis of the angle of rotation θ of the crankshaft 16, as well as the threshold S1 for detection of the rising front and of the falling front of the first tooth T1. FIG. 3 shows the position of the teeth T1, T2, . . . Ti and of the troughs C1, C2. . . Ci of the target 14 with respect to the signal signal B of the magnetic field of FIG. 2.
As illustrated in FIG. 2, in order to determine the position of the crankshaft, the signal B representing the variations of the magnetic field perceived by the sensor 10 of the crankshaft 16 is observed during a revolution of the target 14, i.e. in accordance with an angle of rotation θ of the target 14. This signal has a series of sinusoids D1, D2 . . . Di each corresponding to the variation of the magnetic field measured by the sensor 10 when a tooth T1, T2 . . . Ti (see FIG. 3) followed by a trough C1, C2 . . . Ci passes in front of said sensor 10. By counting the number of sinusoids D1, D2 . . . Di, by measuring the duration of each one thereof, the spacing between each sinusoid D1, D2 . . . Di, and by detecting the missing tooth (the spacing caused by the missing tooth Ce being longer), it is possible to determine the speed of rotation of the engine, the direction of rotation of the engine, and the angular position of the crankshaft.
As illustrated in FIG. 2, the signal B has a minimum BMIN1 and a maximum BMAX1. The passing of the teeth T1, T2 . . . Ti and of the troughs C1, C2 . . . Ci of the target 14 is detected by the detection of the passing of the signal B above (respectively below) a threshold detection S1 placed between the minimum BMIN1 and the maximum BMAX1, for example equal to S1=k1*(BMAX1−BMIN1), k1 being a constant, for example equal to 0.50.
For explanatory purposes, the signal B illustrated in FIG. 2 comprises a minimum threshold BMIN1 and a maximum threshold BMAX1. In reality the signal B has a plurality of minimums BMINi and a plurality of maximums BMAXi and the detection threshold S1 adapts continuously on the basis of the minimums and maximums so as to always be equal to S1=k1*(BMAXi−BMINi). This method for adapting the detection threshold S1 is known to the person skilled in the art, see patent application FR 2 985 035 A1 filed by the applicant, which describes the same method for adapting the detection threshold, but applied to a camshaft sensor.
For the applications of the sensor 10 of the crankshaft 16 on vehicles equipped with the “stop & go” function, i.e. vehicles for which, when at standstill (at traffic lights for example), the engine is stopped temporarily, it is necessary when restarting the vehicle to precisely know the position of the crankshaft. The objective of this constraint is to observe the standards with regard to polluting emissions and to limit the fuel consumption.
When the engine is stopped the crankshaft 16, due to the inertia of said engine, performs a number of movements back and forth before stopping completely. The sensor 10 of the crankshaft 16 therefore is capable not only of increasing the number of teeth and troughs that it detects, but also of decreasing this number.
In addition, during the stopped phase of the engine d (see FIG. 4), which may last a number of minutes, the sensor 10 remains powered and the signal B has a progressive aperiodic drift, i.e. a slope comprising neither a rising front nor a falling front, referred to as thermal drift ΔTAR (see FIG. 4). When the engine is restarted R, the value of the signal B is shifted and has a new minimum BMIN2 and a new maximum BMAX2. It is then necessary to adapt the detection threshold S1 on the basis of these new values BMIN2 and BMAX2 in order to detect, when the engine is restarted, the passing of the third and fourth teeth T3, T4 and of the third and fourth troughs C3, C4. If the detection threshold S1 is not adapted to the new minimum BMIN2 and maximum BMAX2 values, and for example is below the minimum value BMIN2 (as illustrated in FIG. 4), then no tooth (neither T3, nor T4) and no trough (neither C3, nor C4) can be detected during the restart, and the position of the crankshaft cannot be determined.
According to the prior art it is known during the development phase of the sensor 10 to determine an initialization detection threshold SINIT. The initialization detection threshold SINIT is applied from the detection of the rising front and of the falling front of the first tooth T1, during the starting of the engine from cold.
Again according to the prior art, once a maximum value BMAX1 and a minimum value BMIN1 of the magnetic field have been measured by the sensor 10, in other words once the first tooth T1 has passed in front of the sensor 10, a use detection threshold S1′ is then applied. The value of this is S1′=k2*(BMAX1−BMIN1), k2 being a constant between 0 and 1 (k2 can be equal to k1). This use detection threshold S1′ is greater than the initialization detection threshold SINIT and is applied from the detection of the 2nd tooth T2 (rising front or falling front depending on the front that presents itself first).
For a sensor 10 of a crankshaft 16 forming part of a “stop & go” engine, it is known from the prior art, when restarting the engine from warm (detection of the passing of a first tooth), to apply the prior art method described above. In other words it is known to use an initialization detection threshold SINIT, according to the example illustrated in FIG. 4, as the first tooth passes by after the restart, i.e. as the 3rd tooth T3 passes by. Then, after the passing of the third tooth T3, it is known to calculate a utilization detection threshold equal to: S1′=k2*(BMAX2−BMIN2). This new detection threshold S1′ is then applied (in the example illustrated in FIG. 4) with the passing of the 2nd tooth after the restart from warm, this being the rising front of the 4th tooth T4 in the example illustrated in FIG. 4.
However, this method is not reliable when there are vibrations, or oscillations of the crankshaft in the event of restart from warm. These vibrations and these oscillations create extreme values of the signal B that correspond neither to the minimums nor to the maximums of the passing of the third and fourth teeth T3, T4 or of the third and fourth troughs C3, C4 in front of the target 14. This falsifies the calculation of the new threshold detection S1′ and affects the precision of the determination of the position of the crankshaft 16.